Scanning mirror control

Abstract

A scanning beam projection system includes a scanning mirror having a fast-scan axis and a slow-scan axis. Movement on the slow-scan axis is controlled by a slow-scan scanning mirror control system. The control system receives position information describing angular displacement of the mirror. An outer loop of the control system operates in the frequency domain and determines harmonic drive coefficients for a scanning mirror drive signal. An inner loop of the control system operates in the time domain and compensates for a scanning mirror resonant vibration mode at a frequency within the frequency band occupied by the harmonic drive coefficients.

1. A method comprising:
 detecting cyclical movement of a scanning mirror to produce position information; 
 performing a Fourier transform to produce return harmonic coefficients describing frequency content of the position information; 
 comparing the return harmonic coefficients with harmonic coefficient targets to obtain error values; and 
 responsive to the error values, modifying a plurality of drive coefficients at different rates. 
 
     
     
       2. The method of  claim 1  wherein the different rates are specified as static values. 
     
     
       3. The method of  claim 2  wherein the different rates are inversely related to a scanning mirror transfer function. 
     
     
       4. The method of  claim 1  wherein the different rates are adaptively determined. 
     
     
       5. The method of  claim 1  wherein modifying a plurality of drive coefficients at different rates comprises applying a discrete approximation of Newton's method. 
     
     
       6. The method of  claim 1  further comprising:
 producing a time domain waveform from the drive coefficients; and 
 passing the time domain waveform through a compensation circuit to substantially compensate for a resonant mode of the scanning mirror in a frequency band occupied by the drive coefficients. 
 
     
     
       7. The method of  claim 6  further comprising passing the time domain waveform through a filter to remove spectral energy at a resonant frequency outside the frequency band occupied by the drive coefficients. 
     
     
       8. The method of  claim 7  further comprising:
 converting the time domain waveform to an analog drive signal; and 
 driving the scanning mirror with the analog signal. 
 
     
     
       9. A method comprising:
 characterizing a scanning mirror by exciting the scanning mirror at different frequencies and measuring a response; 
 determining a weighting array proportional to an inverse of the scanning mirror response; and 
 using the weighting array to weight learning rates of harmonic drive coefficients in a feedback loop, the harmonic drive coefficients to produce angular displacement of the scanning mirror. 
 
     
     
       10. The method of  claim 9  further comprising operating a second feedback loop in the time domain, the second feedback loop including compensation for a resonant vibration mode of the scanning mirror within a frequency band occupied by the harmonic drive coefficients. 
     
     
       11. A computer-readable medium having instructions stored thereon that when accessed result in a computer performing:
 receiving time domain data describing angular movement of a scanning mirror in a slow-scan direction; 
 performing a fast Fourier transform on the time domain data to produce return harmonic coefficients; 
 comparing the return harmonic coefficients with harmonic coefficient targets to determine errors; 
 modifying a plurality of drive coefficients by differing amounts to reduce the errors; and 
 performing an inverse fast Fourier transform on the drive coefficients to produce a drive waveform to drive the scanning mirror in the slow-scan direction. 
 
     
     
       12. The computer-readable medium of  claim 11  wherein modifying the plurality of drive coefficients by differing amounts comprises applying weights that are inversely related to a frequency response of the scanning mirror. 
     
     
       13. The computer-readable medium of  claim 11  wherein modifying the plurality of drive coefficients by differing amounts comprises adaptively determining weights to apply to each of the plurality of drive coefficients. 
     
     
       14. The computer-readable medium of  claim 13  wherein adaptively determining weights comprises applying a discrete approximation of Newton's method. 
     
     
       15. An apparatus comprising:
 a scanning mirror having a fast-scan axis and a slow-scan axis, the slow-scan axis having a position detector; 
 an outer control loop that operates in the frequency domain to determine harmonic coefficients of a drive signal in response to position information received from the position detector; and 
 an inner control loop that operates in the time domain to compensate for a scanning mirror resonant vibration mode at a frequency within a frequency band occupied by the harmonic coefficients of the drive signal. 
 
     
     
       16. The apparatus of  claim 15  wherein the inner loop includes a bridged-T compensator to compensate for the scanning mirror resonant vibration mode. 
     
     
       17. The apparatus of  claim 15  wherein the inner loop includes filtering to remove spectral energy at one or more resonant frequencies that are outside the frequency band occupied by the harmonic coefficients of the drive signal. 
     
     
       18. The apparatus of  claim 17  wherein the inner loop further comprises a least mean square (LMS) tone cancelling circuit to remove spectral energy corresponding to scanning mirror movement on the fast-scan axis. 
     
     
       19. The apparatus of  claim 15  wherein the outer loop includes an iterative harmonic determination block to modify the drive coefficients by unequal amounts at each iteration. 
     
     
       20. The apparatus of  claim 19  wherein each of the drive coefficients is modified by an amount proportional to an inverse of a scanning mirror frequency response. 
     
     
       21. The apparatus of  claim 19  wherein the iterative harmonic determination block utilizes a discrete approximation of Newton's method to modify each of the drive coefficients. 
     
     
       22. A mobile device comprising:
 a communications transceiver; 
 a scanning mirror; and 
 a slow-scan scanning mirror control system having an outer loop operating in the frequency domain and an inner loop operating in the time domain. 
 
     
     
       23. The mobile device of  claim 22  wherein the outer loop determines scanning mirror harmonic drive coefficients using a discrete approximation to Newton's method. 
     
     
       24. The mobile device of  claim 22  wherein the inner loop includes compensation circuitry to compensate for a scanning mirror resonant vibration mode within a frequency band used to drive a slow-scan axis of the scanning mirror. 
     
     
       25. The mobile device of  claim 22  wherein the inner loop includes a least mean square (LMS) tone cancelling circuit to remove spectral energy at a fast-scan frequency.